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class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">Java复习（二）<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="java-复习-二"><a href="#java-复习-二" class="header-anchor">#</a> java 复习（二）</h1> <p><img src="https://img-blog.csdnimg.cn/0b11358b1b6249828444c829ca5cd16b.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBA5YeM5q2G,size_20,color_FFFFFF,t_70,g_se,x_16#pic_center" alt="在这里插入图片描述"></p> <p>@<a href="%E7%9B%AE%E5%BD%95">toc</a></p> <h2 id="_1-多态"><a href="#_1-多态" class="header-anchor">#</a> 1. 多态</h2> <h3 id="_1-1-多态的前提"><a href="#_1-1-多态的前提" class="header-anchor">#</a> 1.1 多态的前提</h3> <ul><li><strong>要有继承或者实现关系</strong></li> <li><strong>要有方法的重写</strong></li> <li><strong>要有父类引用指向子类对象</strong></li></ul> <h3 id="_1-2-多态中的成员访问特点"><a href="#_1-2-多态中的成员访问特点" class="header-anchor">#</a> 1.2 多态中的成员访问特点</h3> <ul><li><strong>成员方法：编译看左边（父类对象有没有），执行看右边（子类对象有没有）</strong></li> <li><strong>成员变量：编译看左边（父类对象有没有），执行看左边（父类对象有没有）</strong></li></ul> <h3 id="_1-3-向上转型"><a href="#_1-3-向上转型" class="header-anchor">#</a> 1.3 向上转型</h3> <ul><li><strong>父类引用指向子类对象</strong></li></ul> <h3 id="_1-4-向下转型"><a href="#_1-4-向下转型" class="header-anchor">#</a> 1.4 向下转型</h3> <ul><li><strong>子类对象=父类引用</strong></li></ul> <h3 id="_1-5-多态的好与坏"><a href="#_1-5-多态的好与坏" class="header-anchor">#</a> 1.5 多态的好与坏</h3> <p><img src="https://img-blog.csdnimg.cn/8104404c93fe41b3bc102780b7ea7604.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBA5YeM5q2G,size_20,color_FFFFFF,t_70,g_se,x_16#pic_center" alt="在这里插入图片描述"></p> <ul><li><strong>不是重写的方法不能使用，变量不能使用</strong></li></ul> <h2 id="_2-抽象类"><a href="#_2-抽象类" class="header-anchor">#</a> 2. 抽象类</h2> <ul><li><strong>抽象方法：没有方法体的方法</strong></li> <li><strong>抽象类：含有抽象方法的类（可以含有普通方法）</strong></li></ul> <h4 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h4> <ul><li><strong>抽象类中不一定有抽象方法，有抽象方法的一定是抽象类</strong></li> <li><strong>抽象类不能实例化</strong></li></ul> <h3 id="_2-1-抽象类的子类"><a href="#_2-1-抽象类的子类" class="header-anchor">#</a> 2.1 抽象类的子类</h3> <ul><li><strong>要么重写抽象类中的所有抽象方法</strong></li> <li><strong>要么是抽象类</strong></li></ul> <h2 id="_3-接口"><a href="#_3-接口" class="header-anchor">#</a> 3. 接口</h2> <ul><li><strong>一个特定行为的抽象，一个工具，大家在有需要的时候就用，并不是一个公共行为（固有行为）</strong></li> <li><strong>接口本身不能实例化，它的实现类实现了一个接口，这个类在实例化，这叫接口多态</strong></li></ul> <h3 id="_3-1-接口的子类"><a href="#_3-1-接口的子类" class="header-anchor">#</a> 3.1 接口的子类</h3> <ul><li><strong>重写所有抽象方法</strong></li> <li><strong>抽象类</strong></li></ul> <h3 id="_3-2-接口的成员特点"><a href="#_3-2-接口的成员特点" class="header-anchor">#</a> 3.2 接口的成员特点</h3> <ul><li><strong>成员变量：只能是常量，默认修饰符public static final，（不可见）</strong></li> <li><strong>无构造方法</strong></li> <li><strong>成员方法：只能是抽象方法，默认修饰符 public abstract（不可见）</strong></li> <li><strong>一个类可以实现多个接口</strong></li> <li>接口与接口之间是继承关系</li></ul> <h2 id="_4-参数传递"><a href="#_4-参数传递" class="header-anchor">#</a> 4. 参数传递</h2> <h3 id="_4-1-类名作为参数作为形参和返回值"><a href="#_4-1-类名作为参数作为形参和返回值" class="header-anchor">#</a> 4.1 类名作为参数作为形参和返回值</h3> <ul><li><strong>作为形参，传递的是地址值</strong></li> <li><strong>作为返回值，返回的也是地址值</strong></li></ul> <h3 id="_4-2-抽象类作为形参和返回值"><a href="#_4-2-抽象类作为形参和返回值" class="header-anchor">#</a> 4.2 抽象类作为形参和返回值</h3> <ul><li>方法的<strong>形参为抽象类名，起始需要的是该抽象类的子类对象</strong></li> <li>方法的<strong>返回值是抽象类名，其实返回的是该抽象类的子类对象</strong></li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">Animal</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">eat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">Cat</span> <span class="token keyword">extends</span> <span class="token class-name">Animal</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">eat</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;猫吃鱼&quot;</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">AnimalOperator</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">useAnimal</span><span class="token punctuation">(</span><span class="token class-name">Animal</span> a<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">//Animal a = new Cat();</span>
a<span class="token punctuation">.</span><span class="token function">eat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">Animal</span> <span class="token function">getAnimal</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Animal</span> a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Cat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token keyword">return</span> a;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">AnimalDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//创建操作类对象，并调用方法</span>
<span class="token class-name">AnimalOperator</span> ao <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">AnimalOperator</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token class-name">Animal</span> a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Cat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
ao<span class="token punctuation">.</span><span class="token function">useAnimal</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span>;
<span class="token class-name">Animal</span> a2 <span class="token operator">=</span> ao<span class="token punctuation">.</span><span class="token function">getAnimal</span><span class="token punctuation">(</span><span class="token punctuation">)</span>; <span class="token comment">//new Cat()</span>
a2<span class="token punctuation">.</span><span class="token function">eat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br></div></div><h3 id="_4-3-接口名作为形参和返回值"><a href="#_4-3-接口名作为形参和返回值" class="header-anchor">#</a> 4.3 接口名作为形参和返回值</h3> <ul><li>方法的<strong>形参为接口名，其实需要的是该接口的实现类对象</strong></li> <li>方法的<strong>返回值为接口名，返回的该接口的实现类对象</strong></li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">interface</span> <span class="token class-name">Jumpping</span> <span class="token punctuation">{</span>
<span class="token keyword">void</span> <span class="token function">jump</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">JumppingOperator</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">useJumpping</span><span class="token punctuation">(</span><span class="token class-name">Jumpping</span> j<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">//Jumpping j = new Cat();</span>
j<span class="token punctuation">.</span><span class="token function">jump</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">Jumpping</span> <span class="token function">getJumpping</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Jumpping</span> j <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Cat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token keyword">return</span> j;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">Cat</span> <span class="token keyword">implements</span> <span class="token class-name">Jumpping</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">jump</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;猫可以跳高了&quot;</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">JumppingDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//创建操作类对象，并调用方法</span>
<span class="token class-name">JumppingOperator</span> jo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">JumppingOperator</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token class-name">Jumpping</span> j <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Cat</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
jo<span class="token punctuation">.</span><span class="token function">useJumpping</span><span class="token punctuation">(</span>j<span class="token punctuation">)</span>;
<span class="token class-name">Jumpping</span> j2 <span class="token operator">=</span> jo<span class="token punctuation">.</span><span class="token function">getJumpping</span><span class="token punctuation">(</span><span class="token punctuation">)</span>; <span class="token comment">//new Cat()</span>
j2<span class="token punctuation">.</span><span class="token function">jump</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br></div></div><h2 id="_5-内部类"><a href="#_5-内部类" class="header-anchor">#</a> 5. 内部类</h2> <ul><li><strong>定义：在一个类A中定义一个类B</strong></li></ul> <h3 id="_5-1-内部类访问特点"><a href="#_5-1-内部类访问特点" class="header-anchor">#</a> 5.1 内部类访问特点</h3> <ul><li><strong>内部类可以访问外部类的成员</strong></li> <li><strong>外部类的成员访问内部类，需要创建内部类对象</strong></li></ul> <h3 id="_5-2-成员内部类"><a href="#_5-2-成员内部类" class="header-anchor">#</a> 5.2 成员内部类</h3> <ul><li><strong>在成员变量的位置</strong></li> <li>在外界创建成员内部类</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">Outer<span class="token punctuation">.</span>Inner</span> oi<span class="token operator">=</span><span class="token keyword">new</span> <span class="token class-name">Outer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token keyword">new</span> <span class="token class-name">Inner</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><ul><li>一般用法</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">Outer</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">int</span> num <span class="token operator">=</span> <span class="token number">10</span>;
<span class="token keyword">private</span> <span class="token keyword">class</span> <span class="token class-name">Inner</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">show</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>num<span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Inner</span> i <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Inner</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
i<span class="token punctuation">.</span><span class="token function">show</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">InnerDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//Outer.Inner oi = new Outer().new Inner();</span>
<span class="token comment">//oi.show();</span>
<span class="token class-name">Outer</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Outer</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
o<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span>;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><h3 id="_5-3-局部内部类"><a href="#_5-3-局部内部类" class="header-anchor">#</a> 5.3 局部内部类</h3> <ul><li><strong>在方法中定义的一个类</strong></li></ul> <h4 id="特点"><a href="#特点" class="header-anchor">#</a> 特点</h4> <ul><li><strong>外界无法使用，需要在内部创建对象使用</strong></li> <li><strong>该类可以访问外部类的成员和局部变量</strong></li></ul> <h3 id="_5-3-匿名内部类"><a href="#_5-3-匿名内部类" class="header-anchor">#</a> 5.3 匿名内部类</h3> <ul><li><strong>前提：存在一个类或者接口，类可抽象也可具体</strong></li> <li><strong>本质：没有名字的一个类的子类或者一个接口的实现类</strong></li></ul> <h2 id="_6-常用api"><a href="#_6-常用api" class="header-anchor">#</a> 6.常用API</h2> <ul><li><p>Math类</p></li> <li><p>System类</p></li> <li><p>Object类</p></li> <li><p>Arrays类</p></li> <li><p>查阅jdk文档</p></li></ul> <h2 id="_7-object-类的方法重写"><a href="#_7-object-类的方法重写" class="header-anchor">#</a> 7.Object 类的方法重写</h2> <ul><li>toString方法</li> <li>equals方法</li></ul> <h2 id="_8-引用数据类型与包装类型的区别"><a href="#_8-引用数据类型与包装类型的区别" class="header-anchor">#</a> 8.引用数据类型与包装类型的区别</h2> <blockquote><ol><li><strong>引用类型与基本包装类型的主要区别就是对象的生存期</strong></li> <li><strong>使用<code>new</code>操作符创建的引用类型的实例，在执行流离开当前作用域之前都一直保存在内存中</strong></li> <li><strong>自动创建的基本包装类型的对象，则只存在于一行代码的执行瞬间，然后立即被销毁</strong></li></ol></blockquote></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/linxin1123/vuepress-blog/edit/master/docs/02.后端/41.Java笔记/02.Java复习（二）.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2023/02/17, 11:29:32</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/pages/ee64d9/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Java复习（一）</div></a> <a href="/pages/c57687/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Java io流（一）</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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